@import url('https://fonts.googleapis.com/css?family=Muli&display=swap');

* {
    box-sizing: border-box;
}

body {
    /* 水平居中 */
    display: flex;
    justify-content: center;
    align-items: center;
    height: 100vh;

    margin: 0;
    overflow: hidden;
    font-family: 'Muli', sans-serif;
    text-align: center;
    background-color: lightgray;
}

.key {
    /* 行内弹性布局,水平居中,子元素自动换行 */
    display: inline-flex;
    flex-direction: column;
    align-items: center;

    position: relative;
    margin: 10px;
    padding: 20px;
    min-width: 150px;

    font-size: 20px;
    font-weight: bold;
    background-color: #eee;
    border: solid 1px #999;
    box-shadow: 1px 1px 3px rgba(0, 0, 0, .2);
}

.key small {
    position: absolute;
    left: -0;
    top: -24px;
    width: 100%;
    color: #555;
    font-style: 14px;
    text-align: center;
}

@media(max-width:768px) {
    .key {
        margin: 10px 0;
    }
}